Support pad for a drill head and method for designing and manufacturing a support pad

ABSTRACT

A deep hole drill head support pad whose edge chamfers are formed in a continuous grinding operation so that there are no discontinuities in the angled side surface leading up to the outer surface thereof. A method of manufacturing the support pad includes generating a virtual guide metric including a curved travel path for a grinding surface around a support pad blank, and a plurality of control surfaces intersecting with the curved path at separate locations along its length, each control surface defining an angle of orientation of the grinding surface. A CNC grinding machine can interpret the virtual guide metric to change the angle of orientation of a grinding surface relative to a support pad blank as it moves between adjacent control surfaces in a continuous manner.

FIELD OF THE INVENTION

The invention relates to grinding surface profiles of metal objects. Inparticular, the invention relates to the shape and manufacture ofsupport pads for drill heads intended for deep hole drilling, i.e. formachining a hole having a depth of more than 4 times its diameter in aworkpiece of metal.

BACKGROUND TO THE INVENTION

Deep hole drilling is typically performed with drill heads which are notself-centering. Conventional drill heads for deep hole drilling, e.g.using the single tube system (STS) or Ejector system techniques are madewith asymmetrically placed cutting inserts. Such drill heads comprise,at their front end, an outer cutting insert, a inner cutting insert andan intermediate cutting insert. The inner and outer cutting inserts arelocated adjacent to a first chip inlet port and the intermediate cuttinginsert is located adjacent to a second chip inlet port, which isdiametrically opposite the first chip inlet port.

A pair of support pads (also known as guide pads) are mounted on theperiphery of the drill head at its front end. Each support pad presentsa radially projecting surface. These projecting surfaces are intended toabut against a hole wall generated by the outer cutting insert. Togetherwith the outer cutting insert, the support pads provide a three pointcontact to centre the drill head in the hole. The support pads and outercutting insert may thus be spaced from each other around the peripheryof the drill head.

Examples of long hole drills having such support pads are disclosed forexample in U.S. Pat. No. 5,697,737, U.S. Pat. No. 6,602,028 and U.S.Pat. No. 6,682,275.

Conventional support pads are formed as separate objects, e.g. from ahard-wearing material, such as cemented carbide, which are then securedto the drill head, e.g. by brazing or by means of a screw. The supportspads each have a generally cuboidal shape, but where the outer surface(i.e. the surface facing radially away from the drill head to contactthe hole wall) is convex, i.e. describes a convex arch across the widthof the support pad, which is in a circumferential direction of the drillhead. The radially projecting surface presented by the support pad maytaper along the length of the support pad away from the front end of thedrill head, i.e. so that the maximum radial projection is at the frontedge of the support pad.

Chamfers are formed at the edges of the support pad to provide a smoothtransition to the outer surface. A known disadvantage of the chamfers isthat their lines of intersection (typically at the corners of thesupport pad) can be sharp and therefore prone to act as additionalcutting surfaces. This can result in poor quality holes, seen asspirals, ribbing, oversize and score lines on retraction, and can alsoreduce the useful working like of the support pad or drill head and cannegatively affect the surface speed of the drill head in use.

Various attempts have been made to address this disadvantage. Forexample, it is possible to hone the intersections by hand uses a diamondfile or lap. However, by its nature this method is labor intensive andhard to control from a viewpoint of consistency and repeatability.Others have attempted to blend the intersection using additionalconventional machine grinding steps. However, the use of additionalgrinding steps inevitably generating further intersections, since theremust be clearance for lead in and lead off of the grinding surface.

US 2010/0158623 discloses the use of a bespoke grinding step using aconical grinding surface to effectively provide an additional arcingchamfer along the intersection between the front end chamfer and theouter surface of the support pad. However, even in this case furtherintersections are formed where the additional chamber intersects theside chamfers.

SUMMARY OF THE INVENTION

At its most general, the present invention provides a deep hole drillhead support pad whose edge chamfers are formed in a continuous grindingoperation so that there are no discontinuities in the angled sidesurface leading up to the outer (i.e. contact) surface of the supportpad. This is particular advantageous for the corner of the support padbetween its leading side surface and its front surface, since this iswhere the frictional force can be particularly high.

According to a first aspect of the invention, there is provided asupport pad for a drill head of a deep hole drilling machine, thesupport pad having: a outer surface for contacting a work piece to bedrilled, the outer surface describing a convex arch between oppositeside edges thereof; an inner surface opposite the outer surface; aleading side surface between the inner and outer surfaces along a firstside of the support pad; a trailing side surface between the inner andouter surface along a second side of the support pad opposite the firstside; a front surface for locating at the distal end of the drill head,the front surface being between the inner and outer surfaces at a firstend of the leading and trailing side surfaces; a rear surface betweenthe inner and outer surfaces at a second end of the leading and trailingside surfaces opposite the first end; a leading side chamfer between theouter surface and the leading side surface, the leading side chamferbeing oriented at a first angle with respect to the outer surface alonga leading side edge of the outer surface; and a chamfered enteringsurface between the outer surface and the front surface, the chamferedentering surface being oriented at a second angle with respect to theouter surface along a front edge of the outer surface, wherein theleading side chamfer joins the chamfered entering surface via a leadingtransition region whose orientation relative to the outer surfacechanges from the first angle to the second angle without discontinuity.The absence of discontinuity may correspond to a smoothly contouredsurface without any observable lines of intersection between planarsurfaces. According there are no sharp edges at which frictional forcescan become concentrated.

The support pad structure defined above may provide a number ofadvantages. The absence of lines of intersection between differentlyoriented planar surface reduces the friction force on the support pad.In turn this can reduce the torsional load on the drill head, whichhelps to avoid vibration thereby improving the quality of the holes. Inaddition, the working lifetime of the support pad may be increased,which in turn increases the productivity of the drill head as a whole.

Preferably, the leading side chamfer, leading transition region andchamfered entering surface are formed in a single machine grindingoperation. The method by which this can be achieved is set out in detailbelow. Forming the surface in a single operation can be more efficientand repeatable than the known hand honing technique mentioned above. Itmay facilitate efficient industrial scale manufacture of the supportpad.

The support pad may include a trailing side chamfer between the outersurface and the trailing side surface, the trailing side chamfer beingoriented at a third angle with respect to the outer surface along atrailing side edge of the outer surface, wherein the chamfered enteringsurface joins the trailing side chamfer via a trailing transition regionwhose orientation relative to the outer surface changes from the secondangle to the third angle without discontinuity. In other words, the sametechnique can be used on the transition between the chamfered enteringsurface and the trailing side chamfer as between the chamfered enteringsurface and the leading side chamfer, i.e. the chamfered enteringsurface, trailing transition region and trailing side chamfer can beformed in a single machine grinding operation.

In practice it is desirable to have a leading edge chamfer, a chamferedentering surface and a trailing edge chamfer. Preferably all three ofthese chamfers and the leading transition region and trailing transitionregion are formed in a single grinding step with no discontinuities.

The first angle may vary along the leading side chamfer. Any variationis preferably in a continuous manner to avoid the formation of sharpintersections, i.e. discontinuities in the angled surface. Similarly,the second angle may vary along the chamfered entering surface and/orthe third angle may vary along the trailing side chamfer. Again, anyvariation is preferably continuous for the same reasons.

In one embodiment, a recessed portion may be formed substantiallycentrally in the leading edge of the outer surface. The chamferedentering surface may thus include an smoothly indented portioncorresponding to the recessed portion. The centre of the leading edge ofthe outer surface is often the location at which the greatest frictionforces are exerted. Degradation or breakdown of the support pad iscommon at this location. The recessed portion aims to spread thefrictional forces to increase the lifetime of the support pad. Alteringthe geometry of the leading edge of the outer surface was undesirableand in some cases impossible with the conventional support padmanufacturing technique.

A second aspect of the invention provides a method for manufacturing asupport pad as discussed above. Support pads are conventionallymanufactured as cast blanks of material whose final shape is finished ina computer numerical control (CNC) grinding machine, such as the TX7+universal grinder manufactured by ANCA Pty. Ltd., as part of acomputer-aided manufacturing (CAM) process. Conventionally, the supportpad geometry is created using a computer-aided design (CAD) softwarepackage. The normal CAM process includes generating instructions (e.g.G-code) to drive the CNC grinding machine from the CAD representation ofthe support pad. However, the present inventors identified a problemwith this technique, which was that it was extremely difficult toexpress mathematically within a CAD drawings a smoothly contouredchamfered surface around the leading side surface and front surface of asupport pad blank. There was thus an apparent barrier to manufacturingthe desired surface profile, since until it could be expressedmathematically, the relevant instructions to drive the CNC grindingmachine could not be extracted. Moreover, it appeared that a separatemathematical expression would be required for each particular geometry,which would be a burden on manufacture.

The inventors devised a solution to this problem by bypassing thecomplete CAD representation of the support pad. They realised that theCNC machine itself was capable of executing a continuous change inposition of the grinding surface relative to the support pad blank, whenprovided with a start and stop orientation. Thus, the inventors realisedthat if the support pad geometry could be expressed as a skeleton offixed orientations for the grinding surface, the CNC grinding machineitself would provide the smooth transitions. Mathematical expressions ofthose transition thereof did not need to be derived in advance.

Thus, according to the second aspect of the invention, there is provideda method of manufacturing a support pad for the drill head of a deephole drilling machine, the method comprising: generating a virtual guidemetric comprising: a curved path for defining the direction of travel ofa grinding surface around a leading side surface and front surface of asupport pad blank; a plurality of control surfaces intersecting with thepath at separate discrete locations along its length, each controlsurface defining an angle of orientation of the grinding surface at eachdiscrete location on the path; and instructing a computer numericalcontrol (CNC) grinding machine to associate the virtual guide metricwith a support pad blank held in a workpiece holder of the CNC grindingmachine; causing relative movement between a grinding surface of the CNCgrinding machine and the support pad blank held in the workpiece holderaccording to the virtual guide metric; and during the relative movement,changing the angle of orientation of the grinding surface betweenadjacent control surfaces in a continuous manner. Thus, according to themethod of the invention, the virtual guide metric provides incombination a path of movement for the grinding surface relative to thesupport pad blank and a set of positions for the grinding surface on itsway around the path. In interpreting the virtual guide metric, the CNCgrinding machine determines the movement of the grinding surface betweeneach adjacent position. A particular advantage of this technique is thata virtual guide metric may defined by a set of parameters, so thatdifferent geometries can be generated simply by entering a new set ofparameters, rather than generating a entirely new shape.

Thus, the virtual guide metric may include an plurality of parametersstored in a computer memory, the parameters defining properties of thecurved path and control surfaces. The method may include, beforegenerating the virtual guide metric, inputting one or more of theplurality of parameters into the computer memory. For example, thecurved path of the virtual guide metric may comprise a U-shaped path forforming a single chamfered surface around a leading side surface, afront surface and a trailing surface of the support pad blank. TheU-shaped path may be defined by a first set of parameters, e.g. width,length, radius of leading corner, radius of trailing corner, which maycorrespond to the support pad blank intended for use. The plurality ofparameters may include any one or more of: a primary lead angle fordefining the angle of orientation of the grinding surface along acentral portion of the entering surface of the support pad blank; aprimary lead width for defining the width of the chamfer along thecentral portion of the entering surface; a secondary lead angle fordefining the angle of orientation of the grinding surface along theentering surface on each side of the central portion; a secondary leadwidth for defining the width of the chamfer along the entering surfaceon each side of the central portion; a pad leading side angle fordefining the angle of orientation of the grinding surface along theleading side surface of the support pad blank; a pad leading side anglewidth for defining the width of the chamfer along the leading sidesurface of the support pad blank; a pad trailing side angle for definingthe angle of orientation of the grinding surface along the trailing sidesurface of the support pad blank; a pad trailing side angle width fordefining the width of the chamfer along the leading side surface of thesupport pad blank; and a blend radius for defining the radius ofcurvature of the curved path between a leading edge portion and anentering surface portion.

As described above, the support pad blank may include a outer surfacedescribing a convex arch between opposite side edges thereof. In thiscase, the curved path may include an arched portion for following theshape of the outer surface at its leading edge. The radius of the archmay be a further parameter of the virtual guide metric. As mentionedabove, it may be desirable to provide a recessed portion in the frontedge of the outer surface. This can be done by providing a suitableindent in the U-shaped path. The depth and radius of the indent may beparameters of the U-shaped path.

The way in which the CNC grinding machine is arranged (e.g. programmed)to change the angle of orientation between adjacent control surfaces maybe preset in the CNC grinding machine or programmed in separately beforethe virtual guide metric is input. Thus, the step of changing the angleof orientation of the grinding surface may include varying the angle oforientation as the grinding surface travels along the distance x betweenterminal end points of adjacent control surfaces according to acontinuous function φ(x), where

$\frac{\varphi}{x} = 0$

at each terminal end point. The distance x may be along a curvedpathway, e.g. around the leading or trailing corner of the support padblank. The CNC grinding machine may continuous function φ(x) using theconditions and/or boundary conditions provided by the terminal endpoints of the control surfaces and the corresponding derivatives.

In practice, the virtual guide metric may be defined in a CADenvironment, e.g. in relation to a representation of a support padblank. For example, the U-shaped path and the control surfaces mayeffectively define a profile suspended above (i.e. vertically spacedfrom) the representation of the support pad blank. It may be possible tomimic the outcome of the grinding operation by performing a Booleanoperation using the suspended profile and the support pad blank, toprovide a virtual view of the ground surface.

The second aspect of the invention may have applicable outside the fieldof deep hole drilling. It may be useful in any situation where asmoothly contour surface needs to be machined on to an object. Thus, thesecond aspect of the invention may be expressed as a method of applyinga surface profile to an object, the method comprising: generating avirtual guide metric comprising: a path for defining the direction oftravel of a grinding surface along the perimeter of the object; aplurality of control surfaces intersecting with the path at separatediscrete locations along its length, each control surface defining anangle of orientation of the grinding surface at each discrete locationon the path; and instructing a computer numerical control (CNC) grindingmachine to associate the virtual guide metric with the object when heldin a workpiece holder of the CNC grinding machine; causing relativemovement between a grinding surface of the CNC grinding machine and theobject held in the workpiece holder according to the virtual guidemetric; and during the relative movement, changing the angle oforientation of the grinding surface between adjacent control surfaces ina continuous manner.

Embodiments of the invention are discussed in detail below withreference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a conventional deep hole drill head;

FIG. 2 is a front view of the conventional deep hole drill head shown inFIG. 1;

FIG. 3 is a top view of a conventional support pad;

FIG. 4 is a photograph showing a top view of the front end of a supportpad that is an embodiment of the invention;

FIG. 5 is a perspective schematic view of a virtual guide metricsuitable for use in a method that is an embodiment of the invention;

FIGS. 6A, 6B and 6C are a series of views of a support pad of theinvention in which parameters for the virtual guide metric are depicted;

FIG. 7 is a schematic side view of a five-axis CNC grinding machinesuitable for implementing the present invention; and

FIG. 8 is a flow chart showing a method that is an embodiment of theinvention.

FIGS. 1 and 2 show a conventional drill head 10 for a deep hole drillingmachine. The support pad of the invention is suitable for use with drillheads of this type. The drill head 10 comprises a hollow, generallycylindrical body having a rear end 14 (proximal to the drilling machine)and a front end 16 (distal to the drilling machine). In use, the drillhead 10 is mounted on a drill tube (not shown). A male thread 12 on therear end 14 of the drill head 10 is arranged to mate with acorresponding female thread formed in the drill tube.

As shown more clearly in FIG. 2, the front end 16 has threeasymmetrically arranged cutting inserts 18, 20, 22 mounted therein. Thecutting inserts include an outer cutting insert 18 mounted at theperiphery of the drill head 10, an inner cutting insert 20 mountedtowards the centre of the drill head 10, and an intermediate cuttinginsert 22 whose radial distance from drill head axis is between theinner and outer cutting inserts, but which is circumferentially spacedfrom the inner and outer cutting inserts 18, 20. The inner and outercutting inserts 18, 20 are located along a common radius adjacent to afirst chip inlet port 24. The intermediate cutting insert 22 is locatedadjacent to a second chip inlet port 26, which is diametrically oppositethe first chip inlet port 24.

The outer cutting insert 26 provides a radially projecting part 29 (seenmost clearly in FIG. 1) which contacts the hole wall generated duringcutting. A pair of support pads 30 are mounted on the periphery of thedrill head 10 at its front end. Each support pad 30 presents a radiallyprojecting surface which also abut against the hole wall generated bythe outer cutting insert. Together with the outer cutting insert 18, thesupport pads 30 thus provide a three point contact to centre the drillhead in the hole. The support pads and outer cutting insert are spacedroughly equally around the periphery of the drill head 10, althoughother angular arrangements are possible.

FIG. 3 shows one of the support pad 30 in more detail. The support pad30 is mounted in a pocket 32 formed in the side surface of the drillhead 10. The support pad 30 may be fixed in the pocket 32 by soldering.The support pad 30 has a convexly arched outer surface 34. This is thesurface that faces towards the hole wall. The outer surface 34 may taperslightly in the radial direction as the support pad extends away fromthe front end of the drill head. The tapering may be caused by the shapeof the support pad 30 itself, or may be caused by a gradually conicaltapering in the wall of the drill head itself. The front edge of theouter surface 34 terminates at a chamfered entering surface 36.Similarly the leading side edge and trailing side edge of the outersurface 34 terminate at a leading edge chamfer 38 and a trailing edgechamfer 40 respectively. There is also a rear edge chamfer 42 at rearend of the outer surface 34. It can be seen in FIG. 3 that the chamferedentering surface 36 meets the leading edge chamfer 38 and trailing edgechamfer 40 at distinct lines of intersection 44, 46. Similar lines ofintersection occur at where the leading edge chamfer 38 and trailingedge chamfer 40 meet the rear edge chamfer 42. The lines of intersection44, 46 are discontinuities in the angle of the surface around the edgeof the outer surface 34. As explained above, these lines of intersection44, 46 are sharp enough to act as cutting edges as the drill head ismoved into and out of the drilled hole, which can lead to degradation ofthe hole.

The present invention provides a support pad and correspondingmanufacturing method in which the lines of intersection are not present.FIG. 4 shows a top view of a support pad 50 that is an embodiment of theinvention. The support pad is characterised by a continuous chamferaround the leading side edge and front end of the support pad, whichalso continues around to the trailing side edge without anydiscontinuity in angle. Similarly to the conventional support pad 30shown in FIG. 3, the support pad 50 of the invention includes achamfered entering surface 36, a leading edge chamfer 38 and a trailingedge chamfer 40 formed at the front edge, leading edge and trailing edgeof the convexly arched outer surface 34 respectively. The differencefrom the conventional support pad is that instead of the lines ofintersection, the support pad 50 includes a leading transition region 52and a trailing transition region 54. The leading transition region 52comprises an angled surface which extends without discontinuity from theleading side chamfer 38 to the chamfered entering surface 36. Similarly,the trailing transition region 54 comprises an angled surface whichextends without discontinuity from the chamfered entering surface 36 tothe trailing side chamfer 38. Without discontinuity means that the anglesubtended by the chamfered surface relative to a notional cylindricalsurface corresponding to the drill head body varies in a continuousmanner.

The chamfered surfaces discussed above are generated by moving agrinding element, which defines a planar grinding surface, relative to asupport pad blank. The support pad blank may be a cuboidal piece ofmaterial having a convexly arched outer surface. The support pad blankmay be cast or otherwise created in that shape. The grinding element ispart of a CNC grinding machine, preferably a five-axis grinding machinesuch as ANCA's TX7+ universal grinder, which is capable of moving thegrinding surface relatively to the support pad blank (when mounted in aworkpiece holder) with five degrees of freedom.

A schematic drawing of a five axis CNC grinding machine 100 is shown inFIG. 7. The grinding machine 100 has a grinding element 102, which ismounted on a frame 104 that can pivot about a vertical axis 106. Thisenables the orientation of the grinding surface to be altered by onedegree of freedom. A workpiece holder 108 is mounted on a carriage 110.The carriage can move backwards and forwards and side to side relativeto the grinding element, as indicated by arrows 112, 114 respectively.The workpiece holder 108 can moved up and down on the carriage, as shownby arrow 116. Thus, the workpiece holder 108 has freedom of movementrelative to the grinding element in three linear dimensions. Theworkpiece holder can also rotate about a horizontal axis 118.

In the invention, the CNC grinding machine can be instructed to move thegrinding surface relative to the support pad blank in a single grindingoperation to form at least the leading edge chamfer, leading transitionand chamfered entering surface. The single grinding operation may alsoform the trailing transition region and the trailing edge chamfer.

The key to the invention is that the single grinding operation is notinstructed on the basis of a complete CAD representation of the supportblank, but rather is based on a skeleton of reference points (referredto herein as a virtual guide metric) between which the CNC grindingmachine determines a path for the grinding surface. Generating a CADdrawing or model of a support pad with the continuously blended chamfersin a manner that provides future design flexibility, i.e. the ability tovary dimensions, etc., is impractical. The mathematical representationof the chamfered surface, especially in light of the convex nature ofthe front edge of the outer surface, is extremely complex and thereforedifficult to code in software in a straightforward manner. The virtualguide metric, on the other hand, provides a means of parameterising thechamfered surface which allows variations in the angle or shape of thechamfer or in the size of the support pad itself.

FIG. 5 shows a schematic graphical depiction of a virtual guide metric60 suitable for use with the invention. The virtual guide metric 60 iscreated in a CAD environment with respect to a CAD representation of asupport pad blank 62. The virtual guide metric 60 defines a U-shapedcurved guide path 64 and a plurality of control surfaces 66. The curvedguide path 64 defines the route for the grinding surface to take aroundthe top edge of the outer surface of the support pad blank 62. TheU-shaped path thus effectively follows the line of the leading edge,front edge and trailing edge of the support pad blank 62. Each of thecontrol surfaces 66 intersects the curved path 64 at an anglecorresponding to the angle that should be exhibited by the chamfer atthat point on the curved path 64. The curved path 64 lies on a planarprofile 68, which is suspended by a predetermined distance above theouter surface of the support pad blank 62. The predetermined distancemay be a selectable parameter of the virtual guide metric. It may beused by the CNC grinding machine to locate the grinding surface relativeto the support pad blank before grinding occurs. Likewise, each controlsurface 66 may be defined in terms of its angle to the planar profile62.

In the embodiment depicted in FIG. 5 there are nine control surfaces 66.More control surfaces can be used to give greater flexibility over theshape of the chamfer. This may be useful if different distributions ofthe frictional load on the entering surface are required.

The nine control surfaces include a run-in control surface 68 andrun-off control surface 70 at the beginning and end of the U-shaped path64 respectively. These surfaces ensure the grinding surface is properlyaligned with the support pad blank as enters into contact and leaves theblank at the beginning and end of the grinding operation. The leadingedge chamfer and the trailing edge chamfer typically comprise a chamferof constant angle along the side of the support pad blank. The run-inand run-off control surfaces 68, 70 are at this angle. In order tocreate the leading edge chamfer and trailing edge chamfer, the virtualguide metric defines a leading edge terminal control surface 72 and atrailing edge terminal control surface 74, each having the angle oftheir respective chamfer. The run-in control surface 68 and the leadingedge terminal control surface 72 thus provide a pair of control surfaceshaving the same angle. The grinding surface does not need to changeorientation as it moves along the U-shaped path between the controlsurfaces, which means that a chamfer of constant angle is created eventhough the virtual guide metric does not define a graphicalrepresentation of such a surface. The same applied to the run-offcontrol surface 70 and the trailing edge terminal control surface 74.

The leading edge terminal control surface 72 and the trailing edgeterminal control surface 74 intersect with the U-shaped curved path 64at the point where the path starts to curve around the front of thesupport pad blank 62. The radius of the curve may be a parameter of thevirtual guide metric, which in turn may thus affect the position of theleading edge terminal control surface 72 and the trailing edge terminalcontrol surface 74 and hence the length of the leading and trailingchamfers. The radius of the leading edge corner and the radius of thetrailing edge corner may be independent parameters.

In this embodiment, there is a control surface located at the midpointof the leading edge corner and trailing edge corner. The U-shaped path62 includes entering surface portion between the leading edge corner andthe trailing edge corner. This section corresponds to the chamferedentering surface. As this can be the most important part of the supportpad in terms of the frictional forces it experience, the shape may needto be carefully controlled. In this embodiment three front controlsurfaces are provided on this section, e.g. one at the end of theleading edge corner, one at the end of the trailing edge corner and onelocated halfway between those two. These control surface enable thechamfered entering surface to be formed with a varying chamfer, e.g. inthe form of a recessed portion located at the centre of the frontsurface (i.e. at the peak of the convex outer surface).

Upon receiving the parameters which make up the virtual guide metric theCNC grinding machine can plot a path for the grinding surface relativeto the support pad blank (in the workpiece holder) using the controlsurfaces as reference points. The CNC grinding machine can bepre-programmed or instructed to vary the angle of the grinding surfacebetween control surfaces in a constant manner. Essentially this meansthat there should be no discontinuity in the angle of the chamfer,which, if the angle varies between control surfaces with increasingdistance x along the U-shaped path according to a continuous functionφ(x), can be interpreted as requiring

$\frac{\varphi}{x} = 0$

at each control surface. The function φ(x) may be different between eachpair of control surfaces. The CNC machine may have a library offunctions to call on for this purpose. Normally these functions arecalled on when interpreting movements required to reproduce a givenvirtual surface. The invention bypasses the step of representing thesurface graphically, instead replacing the surface more directinstructions for the grinding surface.

FIGS. 6A, 6B and 6C depict various parameters that can form part of thevirtual guide metric. FIG. 6A is a longitudinal cross-sectional viewdown the centre of the support pad. FIG. 6B is a front view of thesupport pad. FIG. 6C is a plan view of the support pad.

FIG. 6A depicts four parameter: a primary lead angle 76 for defining theangle of orientation of the grinding surface along a central portion ofthe entering surface of the support pad blank; a primary lead width 78for defining the width of the chamfer along the central portion of theentering surface; a secondary lead angle 80 for defining the angle oforientation of the grinding surface along the entering surface on eachside of the central portion; and a secondary lead width 82 for definingthe width of the chamfer along the entering surface on each side of thecentral portion. These parameters may thus influence the three frontcontrol surfaces.

FIG. 6B shows four further parameters: a pad leading side angle 84 fordefining the angle of orientation of the grinding surface along theleading side surface of the support pad blank; a pad leading side anglewidth 86 for defining the width of the chamfer along the leading sidesurface of the support pad blank; a pad trailing side angle 88 fordefining the angle of orientation of the grinding surface along thetrailing side surface of the support pad blank; and a pad trailing sideangle width 90 for defining the width of the chamfer along the leadingside surface of the support pad blank. These parameters may thusinfluence the run-in control surface 68, the run-off control surface 70,the leading edge terminal control surface 72, and the trailing edgeterminal control surface 74.

FIG. 6C shows two further parameters: a blend radius 92 for defining theradius of curvature of the curved path between a leading edge portionand an entering surface portion, and a support pad length 94 fordefining the distance between the front of the support pad and the rearedge of the outer surface. The support pad length 94 may be optional.The blend radius 92 may influence the position of the leading edgeterminal control surface 72 and the leading edge corner control surface.

The parameters discussed above are not essential to the virtual guidemetric. Rather they represent an efficient shorthand way of adapting thevirtual guide metric to meet different sizes and shapes of support pad.

FIG. 8 summarizes the steps of a manufacturing technique that is anembodiment of the invention. A first step 120 comprises inputtingparameters in order to generate the virtual guide metric. This step mayencompass drawing the virtual guide metric within a CAD environment, orit may comprise inputting data corresponding to the parameters discussedabove to a system that can generate the virtual guide metric from thoseparameters.

A second step 122 comprises communicating the virtual guide metric tothe CNC grinding machine. This may be done in any conventional manner,similar to the way in which CAD drawings are communicated in a CAMenvironment.

A third step 124 comprises calculating, in the CNC grinding machine, therelative movement of the grinding surface relative to the workpieceholder in accordance with the virtual guide metric. This step mayinclude calculating a function which changes the angle of the grindingsurface varies between adjacent control surfaces in a continuous manner.Given the parameters defining the virtual guide metric, conventional CNCgrinding machines are capable of performing (e.g. can be programmed toperform) such calculations according to known techniques.

A fourth step 126 comprises activating the CNC grinding machine toperform the calculated relative movement in order to manufacture thesupport pad.

1. A support pad for a drill head of a deep hole drilling machine, thesupport pad comprising: an outer surface for contacting a work piece tobe drilled, the outer surface forming a convex arch between oppositeside edges thereof; an inner surface opposite the outer surface; aleading side surface between the inner and outer surfaces along a firstside of the support pad; a trailing side surface between the inner andouter surface along a second side of the support pad opposite the firstside; a front surface for locating at the distal end of the drill head,the front surface being between the inner and outer surfaces at a firstend of the leading and trailing side surfaces; a rear surface betweenthe inner and outer surfaces at a second end of the leading and trailingside surfaces opposite the first end; a leading side chamfer between theouter surface and the leading side surface, the leading side chamferbeing oriented at a first angle with respect to the outer surface alonga leading side edge of the outer surface; and a chamfered enteringsurface between the outer surface and the front surface, the chamferedentering surface being oriented at a second angle with respect to theouter surface along a front edge of the outer surface, wherein theleading side chamfer joins the chamfered entering surface via a leadingtransition region whose orientation relative to the outer surfacechanges from the first angle to the second angle without discontinuity.2. A support pad according to claim 1, wherein the leading side chamfer,leading transition region and chamfered entering surface are formed in asingle machine grinding operation.
 3. A support pad according to claim1, further compising a trailing side chamfer between the outer surfaceand the trailing side surface, the trailing side chamfer being orientedat a third angle with respect to the outer surface along a trailing sideedge of the outer surface, wherein the chamfered entering surface joinsthe trailing side chamfer via a trailing transition region whoseorientation relative to the outer surface changes from the second angleto the third angle without discontinuity.
 4. A support pad according toclaim 3, wherein the chamfered entering surface, trailing transitionregion and trailing side chamfer are formed in a single machine grindingoperation.
 5. A method of manufacturing a support pad for the drill headof a deep hole drilling machine, the method comprising the steps of:generating a virtual guide metric, the virtual guide metric including acurved path for defining a direction of travel of a grinding surfacearound a leading side surface and front surface of a support pad blankand a plurality of control surfaces intersecting with the path atseparate discrete locations along its length, each control surfacedefining an angle of orientation of the grinding surface at eachdiscrete location on the path; instructing a computer numerical control(CNC) grinding machine to associate the virtual guide metric with asupport pad blank held in a workpiece holder of the CNC grindingmachine; causing relative movement between a grinding surface of the CNCgrinding machine and the support pad blank held in the workpiece holderaccording to the virtual guide metric; and during the relative movement,changing the angle of orientation of the grinding surface betweenadjacent control surfaces in a continuous manner.
 6. A method accordingto claim 5, wherein the curved path comprises a U-shaped path forforming a single chamfered surface around a leading side surface, afront surface and a trailing surface of the support pad blank.
 7. Amethod according to claim 5, wherein the support pad blank includes anouter surface following a convex arch between opposite side edgesthereof, and wherein the curved path includes an arched portion forfollowing the shape of the outer surface at its leading edge.
 8. Amethod according to claim 5, wherein changing the angle of orientationof the grinding surface includes varying the angle of orientation as thegrinding surface travels along a distance * between terminal end pointsof adjacent control surfaces according to a continuous function φ(x),where dφ/dx=0 at each terminal end point.
 9. A method according to claim5, wherein the virtual guide metric includes a plurality of parametersstored in a computer memory, the parameters defining properties of thecurved path and plurality of control surfaces.
 10. A method according toclaim 9, wherein the plurality of parameters include any one or morefrom the group of: a primary lead angle for defining the angle oforientation of the grinding surface along a central portion of theentering surface of the support pad blank; a primary lead width fordefining the width of the chamfer along the central portion of theentering surface; a secondary lead angle for defining the angle oforientation of the grinding surface along the entering surface on eachside of the central portion; a secondary lead width for defining thewidth of the chamfer along the entering surface on each side of thecentral portion; a pad leading side angle for defining the angle oforientation of the grinding surface along the leading side surface ofthe support pad blank; a pad leading side angle width for defining thewidth of the chamfer along the leading side surface of the support padblank; a pad trailing side angle for defining the angle of orientationof the grinding surface along the trailing side surface of the supportpad blank; a pad trailing side angle width for defining the width of thechamfer along the leading side surface of the support pad blank; and ablend radius for defining the radius of curvature of the curved pathbetween a leading edge portion and an entering surface portion.
 11. Amethod according to claim 9, further comprising the step of, beforegenerating the virtual guide metric, inputting one or more of theplurality of parameters into the computer memory.
 12. A method ofapplying a surface profile to an object, the method comprising the stepsof: generating a virtual guide metric: the virtual guide metricincluding a path for defining the direction of travel of a grindingsurface along the perimeter of the object; a plurality of controlsurfaces intersecting with the path at separate discrete locations alongits length, each control surface defining an angle of orientation of thegrinding surface at each discrete location on the path; instructing acomputer numerical control grinding machine to associate the virtualguide metric with the object when held in a workpiece holder of thecomputer numerical control grinding machine; causing relative movementbetween a grinding surface of the computer numerical control grindingmachine and the object held in the workpiece holder according to thevirtual guide metric; and during the relative movement, changing theangle of orientation of the grinding surface between adjacent controlsurfaces in a continuous manner